Serialize ActivityPub alternate link into OStatus deletes, handle it (#4730)

Requires moving Atom rendering from DistributionWorker (where
`stream_entry.status` is already nil) to inline (where
`stream_entry.status.destroyed?` is true) and distributing that.

Unfortunately, such XML renderings can no longer be easily chained
together into one payload of n items.

app/workers/pubsubhubbub/raw_distribution_worker.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class Pubsubhubbub::RawDistributionWorker
+  include Sidekiq::Worker
+
+  sidekiq_options queue: 'push'
+
+  def perform(xml, source_account_id)
+    @account       = Account.find(source_account_id)
+    @subscriptions = active_subscriptions.to_a
+
+    Pubsubhubbub::DeliveryWorker.push_bulk(@subscriptions) do |subscription|
+      [subscription.id, xml]
+    end
+  end
+
+  private
+
+  def active_subscriptions
+    Subscription.where(account: @account).active.select('id, callback_url, domain')
+  end
+end